McMaster-Carr offers a selection of snap ring pliers which are recommended by Animal Motor Works and others for installing snap rings into motor casings and motor retainers. AMW recommends the following pliers (by part number) from McMaster-Carr:

Motor Diameter	Part Number
38mm	57805A35 or 5415A63 (0.070" tips)
54mm	57805A36 or 5415A64 (0.090" tips)
75mm	7131A21 (0.108" tips)
98mm	7131A26 (0.120" tips)
150mm	5300A21 (0.120" tips)